Ask anyone who fits floors for a living and they'll tell you the same thing: a hybrid floor is only as good as the subfloor underneath it. The boards click together in an afternoon — it's the prep that makes or breaks the job. Get the subfloor clean, dry and flat and the floor more or less lays itself. Skip it and you'll be fighting gaps, click joints that won't lock and a floor that flexes or peaks for years. Here's exactly how to check your floor, and how to fix it if it needs it.

Step 1: Start with a clean, sound subfloor

Before you measure anything, get the floor properly clean and stable. Sweep and vacuum every last bit of grit — even a small stone trapped under a floating floor will telegraph through and stop the boards sitting flat. Then check the base is sound: on concrete, look for crumbling, dusting or cracks; on timber or particleboard, walk the whole floor and feel for movement and creaks. Screw down any loose boards, repair cracks, and scrape off old adhesive, paint blobs or plaster. You want a hard, stable, smooth base with nothing loose on it.

Step 2: Check for moisture (the step people skip)

Moisture is the silent killer of a floating floor. Hybrid is water resistant on top, but if damp is rising up through a slab it has nowhere to go — it gets trapped underneath and can lead to mould, odours and movement. This matters most on concrete, and especially on new slabs.

New concrete needs time. A fresh slab holds a surprising amount of water. As a rough rule, concrete needs around a month of drying for every 25mm of thickness — so a standard slab often wants 60+ days, longer in cold or humid weather. Don't rush it.

The plastic sheet test (free and easy). Tape a 1m x 1m square of clear plastic sheet tightly to the slab on all four edges and leave it 24–72 hours. Lift it and look: condensation under the plastic, or a darker damp patch on the concrete, means moisture is still coming up and the slab isn't ready. Do this in a few spots, including near external walls where damp shows first.

For an actual number, use a meter. A concrete moisture meter or an in-situ relative humidity probe gives you a reading to check against your flooring's installation guide. If you're at all unsure, it's worth doing — or have a flooring installer test it.

Timber subfloors: make sure there's good airflow underneath (clear the subfloor vents), no plumbing leaks and no damp smell. If the timber feels damp or the space is poorly ventilated, sort that out before you lay.

If you do find a moisture issue, you'll need a moisture barrier (or a slab treatment) before laying. Many hybrid warranties require one over concrete, so check yours.

Step 3: Level vs flat — know which one matters

This trips a lot of people up. A floating floor doesn't actually need to be perfectly level — a floor can gently slope and the boards will be fine. What it needs to be is flat: no humps, dips or sudden steps. So you're not chasing a perfect spirit-level reading across the room; you're hunting for high and low spots.

The standard most manufacturers ask for is around 3mm of variation over a 1 metre span (some allow 4mm over 2m — check your product's guide). Any more than that and the boards will rock, the click joints come under stress, and you'll get gaps or peaking over time.

Step 4: How to check if your floor is flat

There are two reliable ways to find your high and low spots.

The long straightedge or spirit level. The simplest method: lay a long (1.8–2m) spirit level or a straight metal edge across the floor and slide it around the whole room — side to side, end to end and diagonally. Anywhere you can see daylight or slip a coin under the middle is a low spot; anywhere the level rocks on a point is a high spot. Work in a grid so you cover the entire floor, and mark every problem area as you go (a pencil or chalk “H” for high, “L” for low).

The laser level method (best for a whole room). For a clear map of the whole floor, set up a self-levelling laser so it throws a level line around the room (or a rotating laser at a fixed height). Then take a tape measure or a marked stick and measure from the laser line down to the floor at points across a grid — say every 30–50cm — and write each measurement straight onto the floor. The numbers do the talking: where the gap to the laser is smallest you've got a high spot, where it's largest you've got a dip. Circle the worst offenders. This gives you an exact picture of how much to grind down or fill, and where.

Either way the goal is the same: a marked-up floor that shows you exactly what needs fixing before you commit to levelling.

Step 5: Fixing high spots

High spots are usually quicker to deal with than dips. On concrete, grind them back with a concrete grinder and a diamond cup wheel (you can hire one) — work gradually and re-check with your straightedge as you go. Small ridges and trowel marks can sometimes be knocked back by hand. On a timber subfloor, sand the high spots down or plane back a proud board. Re-check flatness once everything's ground back.

Step 6: Fixing low spots and flood-coating (self-levelling)

For the odd small dip, a patching compound troweled in and feathered out is enough. But if the floor is uneven over a larger area — or there are lots of dips — the proper fix is a self-levelling compound, often called a “flood coat” because you pour it across the whole floor and let it find its own level. Here's how it goes:

  • Clean and prime first. The slab must be spotless, then apply the primer made for your levelling compound. The primer stops the slab sucking the water out too fast and stops air bubbles — skipping it is the number one reason self-levelling goes wrong. Let it dry as instructed.
  • Dam the edges and openings. Self-leveller is liquid, so block off doorways and any gaps with timber battens or foam strip so it can't run where you don't want it.
  • Mix it right. Add the powder to the water (not the other way round) and mix with a paddle on a drill to a smooth, lump-free, pourable consistency. Only mix what you can use quickly — most compounds give you just 15–20 minutes of working time.
  • Pour and spread. Pour it out and guide it with a gauge rake or trowel set to your target depth, working from the far corner back towards the door so you don't trap yourself in.
  • Roll out the air. Go over it with a spiked roller while it's still wet to release trapped air bubbles — that gives a smooth, strong finish.
  • Let it cure. It'll be walkable in a few hours, but leave it the full drying time on the bag before you lay (and re-test moisture if you've added a lot of water to the floor).

Do the room in one continuous pour if you can, so it blends into a single flat surface. For deeper areas you may need to build up in layers — stick to the maximum depth on your product.

A note on timber and particleboard subfloors

Timber floors need a slightly different approach. Make sure every board is screwed down tight and not springy, sand back any high joints, and fill small dips with a flexible filler. Standard self-levelling compound isn't always suited to bare timber — if a timber floor is badly uneven, screwing a plywood overlay down over it is often the more reliable fix, giving you a fresh, flat surface to lay on. Always check what your levelling product is rated for before you pour it over timber.

Final checklist before you lay

  • Clean — no grit, dust, adhesive or debris
  • Sound — no loose boards, crumbling concrete or movement
  • Dry — moisture tested and within spec, barrier down if needed
  • Flat — within ~3mm over 1m, high spots ground, dips filled or flood-coated
  • Cured — any compound fully dry before the first board

It's the least glamorous part of the whole job, but it's the part that decides whether your floor still looks perfect in five years or starts lifting in five months. Once your subfloor passes that checklist, you're ready for the fun part — our step-by-step guide to laying hybrid flooring takes it from there.

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